Dimpled heat transfer walls for distillation apparatus

ABSTRACT

According to the present disclosure, a heat transfer wall of a distillation apparatus is dimpled so that a plurality of dimples protrude from the evaporating surface of the heat transfer wall. The dimples are preferrably arranged so that tortuous flow paths are formed between the dimples.

United States Patent Roberts [54] DIMPLED HEAT TRANSFER WALLS FOR DISTILLATION APPARATUS [72] Inventor: Ernest Roth Roberts, Claremont, Calif.

[73] Assignee: Aerojet-General Corporation, El Monte,

Calif.

[22] Filed: Dec. 15, 1969 [21] Appl. No.: 885,116

[52] US. Cl ..202/236, 203/ 10, 203/89, 159/13, 159/28, 138/38 [51] Int. Cl. ..B0ld 3/08, C02b l/04, B0ld 3/28, BOld l/22, BOld 3/00, B01d 1/00 [58] Field of Search ..202/185, 190, 234, 236;

203/10, 11, 27, 89; 165/80, 82, 83, 115, I65, 166; 159/13, 13 X, 15, 24,28; 138/38; 122/182 R, 182

[56] References Cited UNITED STATES PATENTS 3,493,040 2/ 1970 Davidson ..202/236 X 3,291,704 12/1966 Diedrich et a1. ..203/10 X [451 May23, 1972 3,282,797 1 H1 966 Hammer ..202/236 2,445,471 7/1948 3,096,255 7/1963 3,099,607 7/1963 3,175,962 3/1965 3,244,601 4/ 1966 Diedrich ..203/10 X FOREIGN PATENTS OR APPLICATIONS 205,057 3/1955 Australia ..159/13 23,394 l/l883 Germany.... .....l59/28 25,926 4/1883 Germany ..159/24 Primary Examiner-Norman Yudkoff Assistant Examiner-J. Sofer Atlorney-Edward O. Ansell, D. Gordon Angus and T. Reid Anderson [57] ABSTRACT According to the present disclosure, a heat transfer wall of a distillation apparatus is dimpled so that a plurality of dimples protrude from the evaporating surface of the heat transfer wall. The dimples are preferrably arranged so that tortuous flow paths are formed between the dimples.

13 Claims, 13 Drawing Figures Patented May 23, 1972 3,664,928

2 Sheets-Sheet 1 CO/VD EMSA 7'5 Z I COA/CEA/f/QQTE I N VEN TOR. ERA/EST R. ROBE/e75 ATTORNEY Patented May 23, 1972 3,664,928

2 Sheets-Sheet 2 INVENTOR. EBA/E57 2. 20552725 ATTORN EY DIMPLED HEAT TRANSFER WALLS FOR DISTILLATION APPARATUS This invention relates to dimpled heat transfer surfaces, and particularly to distillation apparatus having dimpled heat transfer walls.

A distillation process is one whereby an impure liquid may be purified by vaporizing the liquid and thereafter condensing the vapors to obtain a condensate and a concentrate. For example, fresh water may be separated from saline water in a distillation process by bringing thin films of saline water into contact with a hot surface to vaporize part of the water and separate it from the salt or brine. The vaporized water is then condensed on a cool surface and is recovered as fresh water. Ordinarily, a heat transfer wall separates the saline water from a source of heating fluid, such as steam.

One factor relating to the effectiveness of such distillation apparatus resides in the rate at which the saline water is vaporized per unit area of the heat transfer wall. The rate of vaporization of liquid is dependant, in part, upon the rate at which the heat is transferred to the saline water which in turn is dependant upon the thermal resistance of the heat transfer wall, and the thermal resistance of the layer of saline water on one side of the wall. It is desirable to construct the heat transfer wall from a suitable thermally conductive material, such as copper, and it is desirable to increase the surface area of the wall so that the condensation area and vaporization area are as large as practical.

One problem associated with distillation processes for saline water resided in the fact that the thermal resistance of water is relatively high, and is usually higher than that of the heat transfer wall. Since relatively thin films of liquid transfer their heat more readily than thicker films, it is desirable to maintain both the condensate and the saline water in layers as thin as possible on the heat transfer wall. Heretofore, heat transfer walls for distillation apparatus have been enhanced by providing continuous fins or grooves on one or both sides of the heat transfer wall. These walls, often referred to as fluted or corrugated walls, provided continuous flow paths for condensing and evaporating liquids so that the liquids would develop into streams which run down the surface of the wall. However, prior heat transfer walls providing continuous flow paths for liquid have not been altogether effective for distillation apparatus.

It is an object of the present invention to provide heat transfer walls for distillation apparatus which is more effective than prior walls providing continuous fluid paths.

In accordance with the present invention, distillation apparatus is provided with dimpled heat transfer walls. The dimpled heat transfer walls enhance the overall effective heat transfer surface of the walls. Furthermore, when liquids are distributed on the dimpled surface in sufficient quantities to flood, most of the liquid flows through the low areas between the dimples so that extremely thin films of water are formed over the dimpled portion due to the surface tension of the flood. It is believed that fluid disposed in thin films over the dimpled portion more readily transfers heat than thicker films of liquid thereby achieving condensation of the heating fluid and evaporization of the saline water at a greater rate than heretofore provided by other types of walls. Furthermore, there is no preferential flow path for moisture in a dimpled wall surface so that the film is in a turbulent flow over substantially the entire wall surface of the heat transfer wall. The turbulent flow mixes the concentration of brine which might otherwise occur as a result of evaporation and increases convective heat transfer.

One optional and desirable feature of the present invention resides in the fact that the dimples may be of any configuration, such as spherical or even teardrop, and may be arranged in any desirable pattern.

Another optional and desirable feature of the present invention resides in the use of a shedder or baffle in connection with the dimpled wall to remove excess condensed fluids therefrom.

Another optional and desirable feature of the present invention resides in the arrangement of the dimples so that the flow paths of liquid over the wall surface are tortuous in a vertical direction.

The above and other features of this invention will be more fully understood from the following detailed description and the accompanying drawings, in which:

FIG. 1 is a side view elevation in cutaway cross section of a simplified distillation apparatus in accordance with the present invention;

FIG. 2 is a side view elevation of a dimpled heat transfer wall for use in the apparatus illustrated in FIG. 1;

FIG. 3 is a side view elevation in cutaway cross section of a portion of the dimpled heat transfer wall illustrated in FIG. 2;

FIG. 4 is a side view elevation in cutaway cross section of a modification of the wall illustrated in FIG. 2;

FIGS. 5A-5C are top view elevations of various dimple configurations for heat transfer walls in accordance with the present invention;

FIGS. 6A-6C, 7 and 9 are top view elevations of various dimple patterns for dimpled heat transfer walls in accordance with the present invention; and

FIG. 8 is a section view taken at line 8-8 in FIG. 7.

Referring to the drawings, and particularly to FIG. 1, there is illustrated a housing 10 separated by walls 11 and 12 into chambers 13, 14 and 15. Inlet conduit 16 is provided through a wall of housing 10 to admit saline water into upper chamber 13. Conduit 17 is provided through a wall of housing 10 to admit a heating fluid, such as steam, into chamber 14. By way of example, conduit 16 may be connected to any source of heated liquid or vapor, such as a boiler or the exhaust of a turbine. Outlet conduit 18 permits removal of condensed steam from chamber 14, and outlet conduit 22 permits removal of steam vapor from chamber 14.

Saline water to be distilled is admitted through inlet conduit 16 and permitted to flow, in thin films, down tubes 19,,19a and into lower chamber 15. Tubes 19, 19a, which may be arranged in a bundle, are constructed of suitable heat transfer material. The tubes pass through chamber 14. Outlet conduits 20 and 21 are associated with chamber 15 to remove concentrate (enriched liquid) and evaporate (vaporized water) from chamber 15, respectively. The bundle of tubes may include any number of tubes, the two tubes being shown for sake of clarity.

In FIG. 2 there is illustrated a portion of a heat transfer wall 30 in accordance with the presently preferred embodiment of the present invention. Heat transfer wall 30 may be used for tubes 19, 19a in the distillation apparatus illustrated in FIG. 1. Heat transfer wall 30 is constructed of a suitable heat conductive material, such as copper, copper-nickel allow, copperiron alloy, or aluminum-brass alloy, the particular material used being governed by such factors as durability, thermal conductivity in the range of temperatures contemplated, and availability.

Wall 30 includes a plurality of dimples 31 which are illustrated in greater detail in FIG. 3. Dimples 31, may, for example, be formed in the configuration of a portion of a sphere. In the case of a semi-spherical dimple, the dimple is generated from a center point 29 and has a radius r to the inside surface of the dimple. The dimple has a diameter d across the inside thereof between opposite points where the dimple joins surface 32 of wall 30. Dimension d will be greater than the radius r and less than 2r. Anglea is the angle between opposite portions of the cone generated by radius r as it traces about the circumference of the dimple. It is preferred that angle a be between 60 and The dimple has an inside height sagitta y from an extension of surface 32 of wall 30. Dimension y is less than or equal to radius r. The thickness of wall 30 in the undimpled portion thereof is represented by dimension t and the thickness of the dimpled portion of wall 30 is represented by dimension t As will be observed from an examination of FIG. 3, 2,, is less than dimension t The thickness I of the dimple is proportional to the product of the thickness of the wall t and the ratio of the projected area of the surface to the actual area of the dimple. Hence, the thickness 2,; of the wall forming the dimple can be approximated by the following formula;

where K is a constant. It can therefore be understood as the surface area of the dimple is made larger (and y is made larger) the thickness of the wall forming the dimple becomes thinner.

The dimpled heat transfer walls are preferrably arranged so that the vaporizing surface of tubes 19, 19a has dimples protruding therefrom to form noncontinuous or tortuous flow paths thereon in a vertical direction for both the condensate and the vaporizing liquid. The dimples preferrably protrude into tubes 19, 190, but it is to be understood that the dimples may protrude outwardly instead, or a combination of inwardly and outwardly protruding dimples may be used.

In operation of the distillation apparatus having heat transfer walls in accordance with the present invention, steam is admitted through conduit 17 and contacts the outside or condensing surface of heat transfer tubes 19, 19a. Some of the steam gives up its latent heat of condensation and condenses on the surface of heat transfer tubes 19, 190 at a temperature T (See FIG. 4). The force of gravity on the condensed steam on the outside of the tubes causes the condensed steam to run down the outside walls of the tubes to be discharged through conduit 22 from chamber 14.

Saline water admitted through conduit 16 flows, in a thin film, down the inside wall, or vaporizing surface, of tubes 19, 19a. The temperature of the saline water is at some temperature T below the temperature T of the condensing steam. (See FIG. 4). The saline water is heated and water is vaporized therefrom. The concentrated salt solution or brine continues down the inside of the tube due to the force of gravity and is collected at the bottom of chamber where it is discharged through conduit 20.

The heat transfer capabilities of a wall constructed in accordance with the present invention are significantly greater than heretofore achieved in connection with other types of heat transfer walls for distillation apparatus. It is theorized that when a heat transfer wall is provided with corrugations or flutes in the form of continuous parallel or spiraling grooves, there is a continuous laminar flow of fluid through the grooves and the fluid tends to stratify in the groove and act as an insulator between the heat transfer wall and the bulk vapor. Hence, as steam condenses and as saline water is distributed onto continuous groove-type heat transfer walls, the saline water and the condensate flow in laminar films in the grooves to impose a significantly greater heat transfer resistance between the wall and the vapor. It is theorized that the raised portion of the dimple area is covered with a significantly thinner film of fluid because the surface tension of the fluid pulls the fluid into the depressed portions between the dimples causing it to flow in the depressed paths between the dimples. Hence, any thick flow occuring on the wall will occur only between the dimples, and the dimples cause a turbulent flow. Also, if the dimples are positioned to prevent continuous vertical flow of fluid on the wall, any collection of fluid is divided by dimples downstream, or below the region of thick film formation. Hence, the fluid flow is maintained turbulent.

If the dimples protrude outwardly from the tube, the condensate accumulates and flows through the lower regions of the surface. At the same time, the vaporizing fluid forms thin films on the internal raised portion opposite the depressed portion of the dimples and turbulent films form in the depressions opposite the outwardly protruding dimples. If the dimples protrude inwardly, the thin film is formed on the dimple by the vaporizing liquid, and the condensate forms thin films on the raised portion opposite the inside depressed portions. As illustrated in FIG. 4, some dimples may protrude inwardly while some protrude outwardly so that the advantages of both may be obtained.

The fluid on the raised portion of the dimples is so thin that heat transfer resistance of the fluid on the dimples is relatively low, thereby permitting rapid release of latent heat of condensation by the condensing vapor and rapid absorption of latent heat of vaporization by the evaporating liquid. Furthermore, due to the thinner wall thickness of the dimpled portions of the wall, the heat transfer resistance of the wall is lower in the dimpled portions than the other portions. Since the wall thinning occurs at the same position as where the film of liquid is the thinnest, the heat transfer resistance is minimized and heat transfer capability is maximized. Furthermore, since the depressed portions of the wall, between the dimples, is tortuous, some condensing fluid will shed and fall free when a sufficient flow is established. Hence, when water vapor is condensed onto the outside surface of tubes 19, 19a, excessive condensate falls free from the wall to expose the wall and lower the heat transfer resistance.

When the tubes 19, 19a are constructed in the manner illustrated in FIG. 4, steam is directed to surface 33 of the wall and a layer of condensed steam is condensed thereon at a temperature T A thin layer 35 of saline water to be vaporized is directed onto surface 36 of the heat transfer wall at a temperature T The dimpled portions 37 of the wall are covered with a thinner film of saline water 35 than the non-dimpled portions of the wall. Some dimples 38 are raised on surface 33 of the wall to aid in collection removal of condensed steam from surface 33 of the wall so that the surface tension of the condensate draws the condensate into depressed portions on the wall to develop regions of reduced heat transfer resistance where the condensate film will be relatively thin. Also, baffle 39 may be provided to shed condensed steam from the surface 33 of the wall.

When the tubes 19, 19a are constructed in the manner illustrated in FIG. 4, condensed steam forms on surface 33 as the layer illustrated in 34. When a sufficient quantity of steam has condensed on surface 33 to initiate flooding, it is removed by means of shedder or baffle 39. Likewise, the saline water preferrably flows between dimples 37 on surface 36 of the wall in a tortuous path to induce turbulent flow.

As illustrated in FIGS. 5A, 5B and 5C, the dimples may be in any desired shape. For example, in FIG. 5A dimple 40 comprises a substantially spherical portion whereas in FIG. 5B dimple 41 is somewhat teardropped shaped. In FIG. 5C, dimple 42 is in a shape of a two-way teardrop which is substantially semi-spherical with teardropped tongues at opposite ends. Preferrably, the elongated tongues at opposite ends of the two-way teardrop illustrated in FIG. 5C are arranged in line with the force ofgravity along arrow 44.

FIGS. 6A, 6B and 6C illustrate various patterns of dimples. In FIG. 6A the dimples 43 are arranged in a square or rectangular configuration and are separated by pitch distance P. However, the configuration is off-set from a horizontal line by angle 6. The flow of fluid under the influence of gravity is illustrated by arrow 44. FIG. 68 illustrates a different dimple configuration wherein dimples 45 are arranged in a substantially equilateral triangular grid each separated from the next dimple by a pitch distance P. Like the grid illustrated in FIG. 6A, the grid illustrated in FIG. 6B is off-set from the horizon by angle 6. In FIG. 6C another triangular grid of dimples 46 is illustrated except that the triangular grid is in a form of an isosceles triangle wherein one side P is shorter than the other two sides P, of the triangle. Like the grids illustrated in FIGS. 6A and 68, it is preferred that the grid be off-set from the horizon by some angle 9.

One reason for off-setting the grid from the horizon by angle 6 is to prevent the existence of continuous vertical flow paths for fluid. Thus, by off-setting the grid pattern from the horizon, the dimples become arranged, in a somewhat irregular pattern to a vertical flow path in a direction of arrow 44 so that fluid under the influence of gravity is diverted by the various dimples through a tortuous path between the dimples. For this reason, angle 6 may vary between 0 and depending upon the configuration.

FIG. 7 illustrated another type of grid pattern wherein the dimples are substantially diamond-shaped dimples 47 having flow paths 48 formed between them. FIG. 8 illustrates a cross section of the dimple pattern illustrated in FIG. 7 wherein flow passages 48 are formed in a substantially diamond grid. FIG. 9 is a top view elevation of another type of diamond-shaped grid of dimples 49 having flow channels disposed 45 from the vertical flow path of fluid.

Ordinarily, the shortest pitch P of any arrangement of dimples, as measured between the centers of adjacent dimples, is between about 0.1875 and 1.250 inch. The diameter d across the dimples ordinarily is about 0.125 to 0.750 inch. The inside height y of the dimples is ordinarily between about 0.0084 and 0.3875 inch while the radius r of dimpling is ordinarily between about 0.0625 and 0.3875 inch. Angle a is ordinarily between about 60 and 180. The thickness 2,, of an undimpled wall portion is ordinarily between about 0.020 and 0.650 inch and the thickness t in the dimpled area is ordinarily between about 0.015 and 0.610 inch. The pitch-to-diameter ratio P/d for the smallest pitch in any arrangement is between about 1.06 and 1.66 and the ratio of dimple height to diameter y/d is between about 0.134 and 0.50.

Distillation apparatus having heat transfer walls in accordance with the present invention are more effective in operation than heat transfer walls heretofore used in distillation apparatus and they provide effective maintenance-free operation of the distillation apparatus. The heat transfer walls in accordance with the present invention are easily fabricated and used and are durable.

This invention is not to be limited by the embodiment shown in the drawings and described in the description, which are given by way of example and not of limitation.

What is claimed is:

1. In a distillation apparatus, the improvement comprising: a bundle of vertically disposed rigid, metallic heat transfer tubes, each providing an evaporating surface on one side of the metallic tubular wall and a condensing surface on the other side, an array of a plurality of dimples in said tubular wall and protruding thru the outer surface of the wall to form liquid flow paths thereon between said dimples, said dimples being structurally free of contact with adjacent tubes and their dimples and suitable for the formation of thin liquid films thereover and acting to improve the heat transfer capability of the metallic wall with the wall thickness r of the dimpled portions of said heat transfer wall being less than the wall thickness r of the undimpled portions of said heat transfer wall, a first means for supplying a thin film of a liquid undergoing evaporation to the evaporating surface of said wall, and a second means for supplying a heat-transferring vapor to the condensing side of said wall, and wherein the thickness t,, of the undimpled portions of said wall is between about 0.020 and 0.650 inch and the thickness 2,, of the dimpled portion of said wall is between about 0.015 and 0.610 inch.

2. Apparatus according to claim 1 wherein said dimples are so disposed and arranged in said array that said flow paths are tortuous in a vertical direction.

3. Apparatus according to claim 1 wherein said dimples protrude from said condensing surface of said wall.

4. Apparatus according to claim 1 wherein said dimples protrude from said evaporating surface of said wall.

5. Apparatus according to claim 4 further including a second array of a plurality of second dimples in said wall and protruding thru the inner surface.

6. Apparatus according to claim 1 wherein the inner surface has an array of a second plurality of dimples protruding inwardly therefrom to form second flow paths on said opposite surface between the second dimples.

7. Apparatus according to claim 6 wherein said second plurality of dimples are so disposed and arranged that the flow paths therebetween are tortuous in a vertical direction.

8. In a distillation apparatus, the improvement comprising: a bundle or rigid, metallic heat transfer tubes, each providing an evaporating surface on one side of the tubular wall and a condensing surface on the other side, an array of a plurality of dimples in said tubular wall and protruding thru the outer surface to form liquid flow paths thereon between said dimples, said dimples being structurally free from contact with adjacent tubes and suitable for the formation of thin liquid films thereover and acting to improve the heat transfer capability of the wall with the wall thickness r of the dimpled portions of said heat transfer wall being less than the wall thickness r,, of the undimpled portions of said heat transfer wall and wherein t,, is approximately equal to where y is the inside sagitta of the dimples, r is the radius of the dimpling, and K is a constant.

9. In a distillation apparatus, the improvement comprising: a bundle of rigid, metallic heat transfer tubes, each providing an evaporating surface on one side of the tubular walls and a condensing surface on the other side, an array of a plurality of dimples in said tubular wall and protruding thru the exterior surface to form liquid flow paths thereon, said dimples being structurally free from contact with adjacent tubes and suitable for the formation of thin liquid films thereover and acting to improve the heat transfer capability of the wall with the wall thickness t,, of the dimpled portions of said heat transfer wall being less than the wall thickness t,, of the undimpled portions of said heat transfer wall and wherein the smallest pitch P between adjacent dimples is between about 0.1875 and 1.250 inch, the inside sagitta y of each dimple is between about 0.0084 and 0.3875 inch, and the dimpling radius r is between about 0.0625 and 0.3875 inch.

10. Apparatus according to claim 9 wherein the thickness 2,, of the undimpled portions of said wall is between about 0.020 and 0.650 inch and the thickness r of the dimpled portion of said wall is between about 0.015 and 0.610 inch.

1 1. In a distillation apparatus, the improvement comprising: a bundle of rigid, metallic heat transfer tubes, each providing an evaporating surface on one side of the tubular wall and a condensing surface on the other side, an array of a plurality of dimples in said tubular wall and protruding radially outwardly from the exterior surface to form liquid flow paths thereon, said dimples being structurally free of contact with adjacent tubes and their dimples and suitable for the formation of thin liquid films thereover and acting to improve the heat transfer capability of the wall with the surface opposite the first surface having an array of a second plurality of dimples protruding radially inwardly therefrom to form second flow paths on said opposite surface between the second dimples and wherein the thickness of r of the dimpled portions of the heat transfer wall is less than the wall thickness t of the undimpled portions of said wall and wherein 1),, is approximately equal to l M (1 and where y is the inside sagitta of the dimples, r is the radius of the dimpling and K is a constant.

12. In a distillation apparatus the improvement comprising: a bundle of rigid, metallic heat transfer tubes providing an evaporating surface on one side of the tubular wall and a condensing surface on the other side, an array of a plurality of dimples in said tubular wall and protruding radially outwardly from the outer surface to form liquid flow paths thereon between said dimples, said dimples being structurally free from contact with adjacent tubes and their dimples and suitable for the formation of thin liquid films thereover and acting to improve the heat transfer capability of the wall and wherein the inner surface has an array of a second plurality of dimples protruding radially inwardly therefrom to form second flow paths on said inner surface between the second dimples and wherein the smallest pitch P between adjacent dimples is between about 0.1875 and 1.250 inch, the inside sagitta y of each dimple is between about 0.0084 and 0.3875 inch, and the dimpling radius r is between about 0.0625 and 0.3875 inch.

heat transfer capability of the wall and wherein the interior surface has an array of a second plurality of dimples protruding radially inwardly therefrom to form second flow paths on said interior surface between the second dimples and wherein the thickness t of the undimpled portions of the wall is between about 0.020 and 0.650 inch and the thickness r of the dimpled portion of the wall is between about 0.015 and 0.610 inch, r being less than t IIK 4K 

1. In a distillation apparatus, the improvement comprising: a bundle of vertically disposed rigid, metallic heat transfer tubes, each providing an evaporating surface on one side of the metallic tubular wall and a condensing surface on the other side, an array of a plurality of dimples in said tubular wall and protruding thru the outer surface of the wall to form liquid flow paths thereon between said dimples, said dimples being structurally free of contact with adjacent tubes and their dimples and suitable for the formation of thin liquid films thereover and acting to improve the heat transfer capability of the metallic wall with the wall thickness td of the dimpled portions of said heat transfer wall being less than the wall thickness tu of the undimpled portions of said heat transfer wall, a first means for supplying a thin film of a liquid undergoing evaporation to the evaporating surface of said wall, and a second means for supplying a heat-transferring vapor to the condensing side of said wall, and wherein the thickness tu of the undimpled portions of said wall is between about 0.020 and 0.650 inch and the thickness td of the dimpled portion of said wall is between about 0.015 and 0.610 inch.
 2. Apparatus according to claim 1 wherein said dimples are so disposed and arranged in said array that said flow paths are tortuous in a vertical direction.
 3. Apparatus according to claim 1 wherein said dimples protrude from said condensing surface of said wall.
 4. Apparatus according to claim 1 wherein said dimples protrude from said evaporating surface of said wall.
 5. Apparatus according to claim 4 further including a second array of a plurality of second dimples in said wall and protruding thru the inner surface.
 6. Apparatus according to claim 1 wherein the inner surface has an array of a second plurality of dimples protruding inwardly therefrom to form second flow paths on said opposite surface between the second dimples.
 7. Apparatus according to claim 6 wherein said second plurality of dimples are so disposed and arranged that the flow paths therebetween are tortuous in a vertical direction.
 8. In a distillation apparatus, the improvement comprising: a bundle or rigid, metallic heat transfer tubes, each providing an evaporating surface on one side of the tubular wall and a condensing surface on the other side, an array of a plurality of dimples in said tubular wall and protruding thru the outer surface to form liquid flow paths thereon between said dimples, said dimples being structurally free from contact with adjacent tubes and suitable for the formation of thin liquid films thereover and acting to improve the heat transfer capability of the wall with the wall thickness td of the dimpled portions of said heat transfer wall being less than the wall thickness tu of the undimpled portions of said heat transfer wall and wherein td is approximately equal to where y is the inside sagitta of the dimples, r is the radius of the dimpling, and K is a constant.
 9. In a distillation apparatus, the improvement comprising: a bundle of rigid, metallic heat transfer tubes, each providing an evaporating surface on one side of the tubular walls and a condensing surface on the other side, an array of a plurality of dimples in said tubular wall and protruding thru the exterior surface to form liquid flow paths thereon, said dimples beIng structurally free from contact with adjacent tubes and suitable for the formation of thin liquid films thereover and acting to improve the heat transfer capability of the wall with the wall thickness td of the dimpled portions of said heat transfer wall being less than the wall thickness tu of the undimpled portions of said heat transfer wall and wherein the smallest pitch P between adjacent dimples is between about 0.1875 and 1.250 inch, the inside sagitta y of each dimple is between about 0.0084 and 0.3875 inch, and the dimpling radius r is between about 0.0625 and 0.3875 inch.
 10. Apparatus according to claim 9 wherein the thickness tu of the undimpled portions of said wall is between about 0.020 and 0.650 inch and the thickness td of the dimpled portion of said wall is between about 0.015 and 0.610 inch.
 11. In a distillation apparatus, the improvement comprising: a bundle of rigid, metallic heat transfer tubes, each providing an evaporating surface on one side of the tubular wall and a condensing surface on the other side, an array of a plurality of dimples in said tubular wall and protruding radially outwardly from the exterior surface to form liquid flow paths thereon, said dimples being structurally free of contact with adjacent tubes and their dimples and suitable for the formation of thin liquid films thereover and acting to improve the heat transfer capability of the wall with the surface opposite the first surface having an array of a second plurality of dimples protruding radially inwardly therefrom to form second flow paths on said opposite surface between the second dimples and wherein the thickness of td of the dimpled portions of the heat transfer wall is less than the wall thickness tu of the undimpled portions of said wall and wherein td is approximately equal to where y is the inside sagitta of the dimples, r is the radius of the dimpling and K is a constant.
 12. In a distillation apparatus the improvement comprising: a bundle of rigid, metallic heat transfer tubes providing an evaporating surface on one side of the tubular wall and a condensing surface on the other side, an array of a plurality of dimples in said tubular wall and protruding radially outwardly from the outer surface to form liquid flow paths thereon between said dimples, said dimples being structurally free from contact with adjacent tubes and their dimples and suitable for the formation of thin liquid films thereover and acting to improve the heat transfer capability of the wall and wherein the inner surface has an array of a second plurality of dimples protruding radially inwardly therefrom to form second flow paths on said inner surface between the second dimples and wherein the smallest pitch P between adjacent dimples is between about 0.1875 and 1.250 inch, the inside sagitta y of each dimple is between about 0.0084 and 0.3875 inch, and the dimpling radius r is between about 0.0625 and 0.3875 inch.
 13. In a distillation apparatus the improvement comprising: a bundle of rigid, metallic heat transfer tubes, each providing an evaporating surface on one side of the tubular wall and a condensing surface on the other side, an array of a plurality of dimples in said tubular wall and protruding radially outwardly from outer surface to form liquid flow paths thereon between said dimples, said dimples being structurally free of contact with adjacent tubes and their dimples and suitable for the formation of thin liquid films thereover and acting to improve the heat transfer capability of the wall and wherein the interior surface has an array of a second plurality of dimples protruding radially inwardly therefrom to form second flow paths on said interior surface between the second dimples and wherein the thickness tu of the undimpled portions oF the wall is between about 0.020 and 0.650 inch and the thickness td of the dimpled portion of the wall is between about 0.015 and 0.610 inch, td being less than tu. 